Thin film transistors (TFTs) with organic active layers and printed electronic components are emerging as an inexpensive alternative to silicon-based TFTs for some applications. The organic materials provide the ability to produce these devices without the expensive steps associated with silicon processing. Other advantages include the greater mechanical flexibility and easier tunability of organic-based devices relative to thin film transistors formed on silicon substrates. Although the performance of the organic-based devices will not match the performance of the silicon-based TFTs in terms of device density and reliability under extreme conditions (e.g. high or low temperatures), organic-based TFTs have application where device density and reliability under extreme conditions can be sacrificed in favor of economy.
A variety of organic materials have been proposed for use as the semiconductor material in the organic based TFTs. For example, a variety of p-channel (hole transporting) materials that are characterized as linear, conjugated molecules have been proposed as semiconductor materials for TFTs. Thiophene oligomers are proposed as organic semiconductor materials in Garnier, F., et al., "Structural basis for high carrier mobility in conjugated oligomers" Synth. Met., Vol. 45, p. 163 (1991). Benzodithiophene dimers are proposed as organic semiconductor materials in Laquindanum, J., et al., "Benzodithiophene Rings as Semiconductor Building Blocks" Adv. Mater., Vol. 9, p. 36 (1997). Pentacene is proposed as an organic semiconductor material in Laquindanum, J., et al., "Morphological Origin of High Mobility in Pentacene Thin-Film Transistors" Chem. Mater., Vol. 8, p. 2542 (1996). Other organic semiconductor materials that have hole-transporting characteristics and have been proposed for use as organic semiconductors in TFTs include phthalocyanines described in Bao, Z., et al., "Organic field-effect transistors with high mobility based on copper phthalocyanine" Appl. Phys. Lett. Vol. 69, p. 3066 (1996) and poly(alkylthiophenes) described in Bao, Z., et al., "Soluble and processable regioregular poly(3-hexylthiophene) for thin film field-effect transistor applications with high mobility" Appl. Phys. Lett. Vol. 69, p. 4108 (1996). Although these materials exhibit hole mobilities which make them attractive as potential organic semiconductors for TFT devices, some improvement in the properties of these materials is required in order for TFT devices with organic semiconductors to be commercially feasible. An attractive material would have a high mobility (i.e. a mobility at least above 0.01 cm.sup.2 /V-s with a low, zero-field conductivity (i.e. &lt;10.sup.-6 S/cm). An attractive film would also be deposited from the liquid phase, have a thermal stability of at least 200.degree. C., and have stability to oxidation and photo-oxidation.
For example, pentacene is reported to have a mobility of about 1 cm.sup.2 /V-s. However pentacene films are difficult to form from solution because pentacene is extremely insoluble in organic solvents. Pentacene also tends to oxidize over time in an oxygen-containing atmosphere and thus exhibits what is referred to as oxidative instability.
Accordingly, although numerous materials have been proposed for use as the organic semiconductor for TFTs, organic semiconductor materials that exhibit all of the above characteristics are desired.